A number of different types of ethylene polymers are presently known and in widespread use. Thus, high pressure (low density) polyethylene produced by the free radical polymerization of ethylene is generally characterized by a branched chain structure containing a variety of types of short chain as well as long chain branches. High density polyethylene, produced by the polymerization of ethylene under relatively low pressures using catalysts comprizing mixtures of transition metal compounds and aluminum alkyls, are generally linear in structure, lacking side chain branches. Its density is usually at least 0.940, with homopolymers generally having relative densities of at least 0.960. Another type of ethylene polymer whose commercial production is now well established is linear low density polyethylene (LLDPE) which is a copolymer of ethylene with minor amounts, usually less than 20%, of a higher alpha-olefin such as butene, hexene, methylpentene, octene or decene. LLDPE is produced by the polymerization of ethylene in the presence of the requisite comonomer using a catalyst of the transition metal/aluminum alkyl type and the product is characterized by a linear structure, having long sequences of methylene units with periodic, uniform side chains distributed statistically along the molecular chain, whose nature depends upon the identity of the comonomer. The short chain branching interferes with crystallization of the main chains and since the crystalline regions display a higher density than noncrystalline regions, the ramification lowers the density. The density of LLDPE is generally below 0.940, usually in the range 0.910 to 0.940 (all densities referred to in this specification are relative densities).
Linear low density polyethylene has a number of properties which make it superior to conventional high pressure polyethylene (HP-LDPE) at similar melt indices and densities. LLDPE will generally possess higher tensile strength, flexural modulus, better elongation and stress-crack resistance. However, the rheology of molten LLDPE differs from that of conventional high pressure polyethylene and at comparable melt indices and densities, LLDPE displays a higher viscosity that decreases less rapidly with increasing shear stress than that of a high pressure (low density) polyethylene.
In general terms, the degree of viscosity decrease depends upon the molecular weight, molecular weight distribution and molecular conformation of the polymer. Broad molecular weight distribution and long chain branching are related to enhaced shear thinning behavior in the shear rate range used in resin extrusion. These factors combine so that conventional high pressure, low density polyethylene (HP-LDPE) which commonly has a relatively broad molecular weight distribution as well as long chain branching, exhibits a more marked viscosity decrease under extrusion shear conditions than LLDPE which has only short chain branching and a relatively narrow molecular weight distribution. Other differences between high pressure, low density polyethylene (HP-LDPE) and LLDPE are found in the behavior of the polymer melt during extension. The extensional viscosity of high pressure, low density polyethylene increases with increasing strain rate and the resin is said to be strain hardening. By contrast, LLDPE shows a markedly different behavior with relatively little strain hardening, at least at low strain rates. These differences in rheology may be expressed in simple terms by stating that compared to high pressure, low density polyethylene (LDPE), the linear, low density polymer (LLDPE) is stiff in shear and soft in extension. These and other differences between these polymers are described in greater detail in U.S. Pat. No. 4,243,619 and in the article "Film Extrusion of Low Pressure LDPE"; Fraser, W. A. et al., TAPPI 1980 (1980 TAPPI Paper Synthetics Course Proceedings), to which reference is made for such details.
One consequence of the rheological differences is that equipment used to fabricate high pressure (low density) polyethylene may be unsuitable in some instances for processing LLDPE at commercially acceptable rates. Although this problem may be wholly or partly overcome by making LLDPE resins to a higher melt index than their high pressure counterparts, it would be nonetheless desirable to improve the properties of LLDPE so that it could be fabricated more readily on conventional processing equipment.
One type of fabrication process which may be used with the various types of polyethylene is the blown film process in which a molten tube of the resin is extruded, generally in a vertically upwards direction to a set of nip rolls, usually about 2 to 7 m. above the extrusion die. A free standing bubble is formed by the injection of air into the interior of the bubble, this internal pressure serving to expand the tube by about two to five times its original diameter and to confer a transverse direction (TD) molecular orientation on the film. At the same time, a longitudinal or machine direction (MD) orientation is created by taking up the cooled film through the nip rolls at a speed greater than the extrusion speed. After passing through the nip rolls the film may be cooled further and rolled up or passed to further processing steps, e.g. slitting and forming. The blown film process is described in further detail in Encyclopedia of Chemical Technology, Kirk-Othmer, Third Edition, John Wiley & Sons, New York 1981, Vol. 16, pp 416-417 and Vol. 18, pp 191-192, to which reference is made for details of the process.
One particular problem encountered in the fabrication of films by the blown tubular film process is that the strain hardening behavior of high pressure, low density polyethylene imparts good bubble stability to the process; the polymer is said to have good melt strength and the film may be extruded at commercially satisfactory speeds (it should, however, be noted that the same strain hardening behavior may restrict the level of drawdown which can be achieved since the melt is capable of showing unbounded stress growth during elongation and for this reason, relatively narrow die gaps are used so that the desired final film gauge can be achieved without excessive drawdown). The linear polymer, by contrast, does not have such good bubble stability because it does not exhibit the same strain hardening behavior, i.e. it does not have good melt strength. This imposes a constraint upon the speed with which LLDPE can be extruded into a film and generally, it has been found necessary to operate LLDPE blown film operations at speeds which are lower than those which can be used with the high pressure polymer. Similar constraints may also arise in slot cast film production, again limiting the extrusion speeds which can be commercially employed. This is obviously undesirable from the economic point of view; it would therefore be desirable to increase the melt strength of LLDPE in order to permit it to be made into film at higher speeds. Furthermore, if the crystallization rate could be enhanced this would also be advantageous because higher line speeds will tend to move the frost line in blown film extrusion higher up the bubble.
Other differences between the properties of the polymers may also be noted when they are fabricated with finished products. For instance, one characteristic of LLDPE and certain other polyethylene films is that they tend to block or adhere together in a non-permanent but relatively strong bond after the films have been in face-to-face contact under pressure for some time. Rolls of the film may be difficult to unwind if the take-up tension has been great enough to cause a significant amount of blocking to take place and surface imperfections may be created as the roll is unwound. Although the phenomenon of blocking may be put to use in certain laminate products, it would be desirable to have some means of reducing the blocking tendencies of the films.
Proposals have been made for improving the properties of LLDPE in various ways, for example, by treatment with organic peroxides, as described in U.S. Pat. No. 4,460,750. In this case, the improvements achieved are stated to be in the transparency of the film. Treatment with peroxides has, however, the disadvantage that various properties of the film do suffer, notwithstanding assertions to the contrary in U.S. Pat. No. 4,460,750. In particular, the mechanical strength of the film, as measured by the elongation, toughness and tear strength, have been found to be undesirably low following treatment with peroxides. Other treatments for LLDPE using peroxides and unsaturated silane compounds to promote cross-linking in order to form improved wire coating materials are described in U.S. Pat. Nos. 4,320,214, 4,289,860, 4,252,906, 4,228,255, 4,117,195 and 3,646,155.
Additives for improving the antiblocking properties of films are known and are commercially available. They are members of a large family of parting agents known as abherents and in the polyolefin field, are often referred to as slip agents. They may be chosen from a wide variety of materials including natural and synthetic waxes, fatty acid salts and various polymers and inorganic compounds such as silica and silicates. However, these additives are generally single purpose materials which do not usually have any other significant effect upon the film properties.